This invention relates to a nuclear reactor fuel element. More specifically this invention relates to a nuclear reactor fuel element having a layer of oxygen gettering material on the inside surface of the cladding. This invention also relates to an improved method for coating the inner surface of the cladding with a layer of oxygen gettering material.
A continuing supply of fissionable material is necessary to fuel future nuclear power plants to ensure adequate electrical power to meet the needs of the future. At present, power reactors are fueled with fissionable uranium-235 of which only a limited supply is available. To overcome this shortage of fissionable material, "breeder" power reactors are being developed, which produce more new fissionable material than consumed in sustaining the reaction. For example, fissionable .sup.233 U or .sup.239 Pu is bred from fertile .sup.232 Th or .sup.238 U which is relatively abundant.
Because of their desirable physical characteristics, a reactor fuel of mixed plutonium and uranium oxides is being considered to fuel the breeder reactors presently under development. However, several problems have been discovered which are associated with the use of the mixed oxides. For example, it has been found that mixed oxide fuels are far more oxidizing than uranium oxide when used alone as a fuel. This oxidizing power is also known as the oxygen potential and is a measure of the driving force for the numerous reactions which take place in the fuel element during irradiation. Among other problems, this oxidizing potential provides the chemical driving force for corrosive attack of the fuel element cladding, controls the vapor pressure of many fuel components, especially that of the uranium oxides and thus redistribution of uranium in the mixed oxide fuel matrix, controls the chemical state of many fission products, their interaction with the fuel contributes to fuel swelling, volatility and redistribution.
A particular problem has been the attack by the oxygen upon the fuel element cladding. Two types of cladding attack have been observed at the fuel-cladding interface. One is a general recession of the cladding thickness by a uniform oxidation of the stainless steel. The second is intergranular penetration by oxygen and fission products along grain boundaries in the cladding. The fission products cesium, molybdenum, tellurium and iodine are also significant factors in influencing the degree and type of cladding attack.